1. Field of the Invention
The present invention is broadly concerned with an improved method of preparing roofing granules which effects a permanent color change that resists fading. More particularly, it is concerned with a method in which roofing granules are prepared from granulated mineral-containing matter having an iron content of about 15% to about 70% by weight to yield buff, brown or reddish-brown color. The granules are heated to a temperature of from about 450.degree. C. to about 1,000.degree. C. for a period of time sufficient to effect a color change.
2. Description of the Prior Art
Roofing granules are commonly used as a surface coating for composition substrate roofing materials. In conventional methods, pigments are applied in order to achieve popular color shades. Typically, the granules are coated with pigments in inorganic binders in the presence of clay or other fillers and reactants.
Once applied as a surface coating to composition roofing material and installed on a roof, such colored granules may have a tendency to fade. After exposure to the elements over a period of time, the extent of such fading may be of sufficient magnitude to preclude aesthetically satisfactory patching of the roof with new materials.
The method of the present invention utilizes the chemical composition of the granules to produce a permanent color change through heat treatment without the use of artificial dyes or pigments.